In many countries there is a short supply of water and it is costly to construct sewers of an appropriate slope to use water as a transporting medium. By choosing to have an early separation the transport is facilitated and the need of water is reduced. A lower amount of water will also facilitate an efficient treatment of sludge. By constructing a separating device as close as possible to the origin of the waste water the problems discussed above can be reduced and the quality of the sludge improved.
Such an early separation is described in SE 9000921 according to which a sewer system having a plurality of separating tanks is used.
To use digestion of sludge for reducing biological material and odor in the treatment of sludge is well known from a plurality of patent documents. Such use is described e.g. in SE 7801404 and SE 7909320.
Also DE-A1-3 230 010 disclosed a device for digestion of sludge. A tank divided into four chambers is used for the digestion. The tank can be tipped between two positions. The digestion process takes part in a bottom layer and some sludge will float up to the surface where it is digested only slowly or not digested at all. Biogas which is formed during the process presses liquid from one chamber into the second, and the tank is tipped from slightly tipping in one direction to slightly tipping in another direction. Valves within the tank are used for the passage of gas in an appropriate way. According to the patent document the steering of sludge should be slow and without turbulence, and a plurality of rotatable flaps and similar devices are used. The reliability in operation of the system described in DE-A1-3 230 010 tends to be low as a result of using movable parts and valves within the tank.
Problems that are not considered completely presently when devices for the digestion of sludge are used are e.g. odor and extended required digestion periods resulting in high demands on large sludge drying beds and on a high quality of the sludge and also to the concentration to large plants.
An object of the present invention is to essentially solve the problems discussed above such as large volumes, odor, low quality of sludge and specially made material. Another object is to accomplish a reliable device.
The object has been obtained by including in the invention the features of claim 1.